Such diaphragm springs are generally annular members comprising a continuous, Belleville washer peripheral portion and a central portion divided into a plurality of radial fingers, which diaphragm springs have a generally frustoconical configuration in the rest or unloaded position.
The Belleville washer peripheral portion comprises axially acting resilient means which in the context of a diaphragm clutch bear against the fixed housing called a cover and carry the axially movable member called a pressure plate so that the latter is at all time biased toward its engagement position. At the same time the radial fingers of the central portion comprise release levers which in response to the action of a part called a release bearing are adapted to control the freeing of the pressure plate through the Belleville washer peripheral portion and thereby its displacement from the engaged to the disengaged position.
In practice, in the vicinity of the roots or radially outer ends of the radial fingers and alternating therewith, the diaphragm spring comprises apertures which open radially into slots which separate the radial fingers from one another.
Some of these apertures, referred to herein as the main apertures, are employed for receiving the lugs, tabs, rivets or the like for rockably mounting the diaphragm spring on the cover; the area of the apertures is therefore determined by the cross-sectional area of the corresponding lugs, tabs, rivets or the like.
For practical manufacturing and assembly reasons all the apertures in such a diaphragm spring have up to the present usually been identical and each of the apertures which are not intended to receive the tabs, lugs, rivets and the like defines an area equal to that of those which are actually employed.
One difficulty with the making of such a diaphragm spring results from the fact that in the course of clutch disengagement the radial fingers of the central portion do not have the same angle of inclination as the peripheral portion of the diaphragm spring; in general the angle of inclination of the flexing radial fingers is greater than that of the peripheral portion. This produces less pressure plate lift, i.e., disengagement displacement, for a given release bearing travel and in addition, there is an enhanced possibility of circumferential waving or undulations in the Belleville washer peripheral portion which may cause rupture.
Various attempts to ameliorate this difficulty have been proposed.
For example, in German ALS No. 2, 757,892 additional apertures are provided which are arranged along the same circumference as the apertures receiving the lugs, tabs, rivets or the like which alternate with the same but do not open into slots and have areas substantially equal to those of the apertures utilized. Such an arrangement which comes down to modifying the ratio between the nonperforate portions and the apertures along the circumference concerned, alters the load characteristics of the diaphragm spring.
In French printed patent application No. 2,244,101 the radial fingers are provided with ribs for their rigidification and these ribs may extend into the Belleville washer peripheral portion. But as above, this arrangement which is moreover difficult to carry out, affects, all other things being equal, the load characteristics of the diaphragm spring. Further, in this printed application there is a difference in size of the various apertures but effects produced by this difference are mentioned.
The present invention is based on the observation that differential sizing of the apertures alone is sufficient to obtain satisfactory rigidification of the radial fingers of the diaphragm spring, without any need to rib the radial fingers. This result may be explained by the increased size of some of the radial fingers in the vicinity of their roots or radial outer ends.
On the other hand, it could not have been apparent to one skilled in the art having knowledge of French printed patent application No. 2,244,101 who would normally have attributed the rigidification observed to the ribbing of the radial fingers of the diaphragm spring which is explicitly provided for this purpose.